With an electrophotographic image recorder, it has been customary to improve and stabilize the quality of image recording by measuring the surface potential of a photoconductive element, or image carrier, immediately after the surface of the photoconductive element has been charged by a charger, comparing the measured potential with a predetermined reference value, and so controlling the output of the charger, the amount of imagewise exposure and so forth so as to maintain the surface potential constant.
After a toner image has been transferred from the surface of the photoconductive element to a paper sheet or similar material, toner particles remains on the photoconductive element. It is a common practice to remove the remaining toner particles by discharging the surface of the photoconductive element after the image transfer and then sweeping away the particles by use of a magnet brush to which a bias voltage is applied.
In general, the cleaning ability available with a system of the kind using a bias voltage as stated above is greatly effected by an electric field acting during the course of cleaning, (i.e., an electric field developed by the surface potential of the photoconductive element after the discharging which precedes cleaning and the bias voltage applied to the magnet brush). Specifically, effective cleaning is achievable only when the cleaning electric field lies in a certain range. Otherwise, the cleaning ability is lowered to invite an occurrence that toner particles adhere to the photoconductive element after the cleaning. The cleaning electric field, or surface potential of the photoconductive element after discharging, often varies over a wide range with the degree of contamination and deterioration of a wire of a discharger, the aging of the photoconductive element, etc. It is therefore necessary to adequately control the surface potential of the photoconductive element after the discharging so that the cleaning ability may be improved. A cleaning system which implements such a manner of control has not been proposed yet.